1. Field of the Invention
The present invention relates to an antenna coil for transmission, and more particularly, to an antenna coil used in a communication system for a short distance, which utilizes electromagnetic waves of an LF-band.
2. Description of the Related Art
A short-distance communication system utilizing an LF-band (30 kHz to 300 kHz) is primarily used in a keyless entry system for locking and unlocking vehicle doors with a remote control. A transmission antenna coil used in such a system is formed by winding a coil around a bobbin which surrounds a magnetic core, and by placing a unit of wound body in a case. Usually, the transmission antenna coil is built in a door handle or a side mirror, and it supplies electromagnetic waves to a reception antenna coil held by a user.
Japanese Unexamined Patent Application Publication No. 2001-358522 discloses an antenna coil which can be used as a transmission antenna coil in the keyless entry system. FIG. 7 is a perspective view illustrating the structure of the antenna coil described in Japanese Unexamined Patent Application Publication No. 2001-358522. An antenna coil 500 described in Japanese Unexamined Patent Application Publication No. 2001-358522 includes a wound body 504 and a case 502 housing the wound body 504. The wound body 504 includes a magnetic core 506, a bobbin 508 surrounding the magnetic core 506, and a coil 510 wound around the bobbin 508. A potting material 522 is filled in the gap between the wound body 504 and the case 502 by vacuum injection.
In Japanese Unexamined Patent Application Publication No. 2001-358522, the potting material 522 is made of a defoamed body which is produced by removing bubbles. Furthermore, the defoamed body is made of a rubber material having high flexibility to absorb a static deformation and load, which are applied to the case 502, with a deformation of the defoamed body, thus preventing the static deformation and load from being transmitted to the magnetic core 506 through the defoamed body.
However, if the defoamed body is filled between the case 502 and the wound body 504 without leaving gaps therebetween, there is a high probability that, upon a deformation of the case 502 or an application of a load thereto, the defoamed body is not deformed and the deformation of the case 502 or the applied load is transmitted to the magnetic core 506. Also, when the rubber material is used for the defoamed body, the response of the rubber material to a momentary deformation or load is too poor to reliably prevent breakage of the magnetic core 506.
Furthermore, when the defoamed body is filled in the case 502 by vacuum injection, a deformation generated upon curing of the defoamed body may shift the position of the wound body 504 to such an extent that the defoamed body is partially thinned and the ability to absorb the deformation and the load is partially reduced. In other cases, the defoamed body may be cured such that stress is applied to the magnetic core 506. Those phenomena may cause breakage of the magnetic core 506.